The present invention relates to power control in wireless communication systems. More particularly, and not by way of limitation, the present invention is directed to a system and method for controlling transmit power of a Physical Uplink Control Channel (PUCCH) signal in a cellular wireless network with Carrier Aggregation (CA).
In a wireless communication system (e.g., a third generation (3G) or a Long Term Evolution (LTE) fourth generation (4G) cellular telephone network), a base station (e.g., an evolved Node-B or eNodeB (eNB) or a similar entity) may transmit wireless channel resource allocation information to a mobile handset or User Equipment (UE) via a downlink control signal, such as the Physical Downlink Control Channel (PDCCH) signal in Third Generation Partnership Project (3GPP) 3G and 4G networks. Modern cellular networks use Hybrid Automatic Repeat Request (HARQ) in which, after receiving this PDCCH downlink transmission (i.e., transmission from a base station to a mobile device), the UE may attempt to decode it and report to the base station whether the decoding was successful (ACK or Acknowledge) or not (NACK or Negative Acknowledge). Such reporting may be performed by the UE using an uplink transmission (i.e., transmission from a mobile device to a base station in a cellular network), such as the Physical Uplink Control Channel (PUCCH) signal in the 3G and 4G networks. Thus, the uplink control signal PUCCH from the mobile terminal to the base station can include hybrid-ARQ acknowledgements (ACK/NACK) for received downlink data. The PUCCH may also additionally include terminal reports (e.g., in the form of one or more Channel Quality Indicator (CQI) bits) related to the downlink channel conditions. Such reports may be used by the base station to assist it in future downlink scheduling of the mobile handset. The PUCCH may further include scheduling requests by the UE, indicating that the mobile terminal or UE needs uplink resources for uplink data transmissions.
The general operations of the LTE physical channels are described in various Evolved Universal Terrestrial Radio Access (E-UTRA) specifications such as, for example, 3GPP's Technical Specifications (TS) 36.201 (“Physical Layer: General Description”), 36.211 (“Physical Channels and Modulation”), 36.212 (“Multiplexing and Channel Coding”), 36.213 (“Physical Layer Procedures”), and 36.214 (“Physical Layer—Measurements”). These specifications may be consulted for additional reference and are incorporated herein by reference.
It is observed here that LTE Release-8 (Rel-8) now has been standardized to support operating bandwidths of up to 20 MHz. However, in order to meet International Mobile Telecommunications (IMT)-Advanced requirements, 3GPP has initiated work on LTE Release-10 (Rel-10) (“LTE Advanced”) to support bandwidths larger than 20 MHz. One important requirement in LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This includes spectrum compatibility, i.e., an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of (smaller) LTE carriers to an LTE Rel-8 terminal. Each such smaller carrier can be referred to as a Component Carrier (CC). It is observed here that during initial deployments of LTE Rel-10, the number of LTE Rel-10-capable terminals may be smaller compared to many LTE legacy terminals (e.g., Rel-8 terminals). Therefore, it is necessary to assure an efficient use of a wide (Rel-10) carrier also for legacy terminals. In other words, it should be possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. One way to obtain this efficient usage is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where each CC has, or at least the possibility to have, the same structure as an Rel-8 carrier. FIG. 1 illustrates the principle of CC aggregation. As shown in FIG. 1, an operating bandwidth of 100 MHz (indicated by reference numeral “2”) in Rel-10 may be constructed by the aggregation of five (contiguous, for simplicity) smaller bandwidths of 20 MHz (in compliance with Rel-8 requirements) as indicated by reference numerals “4” through “8”. It is noted here that Rel-10 supports aggregation of up to five carriers, each with a bandwidth of up to 20 MHz. Thus, for example, if desired, carrier aggregation in Rel-10 also may be used to aggregate two carriers of 5 MHz bandwidth each. The carrier aggregation in uplink and downlink may thus support higher data rates than possible in legacy communication systems (i.e., UE's operating under 3GPP Rel-8 or below). UE's capable of operating only over a single Downlink/Uplink (DL/UL) pair may be referred to as “Legacy UE's”, whereas UE's capable of operating over multiple DL/UL CCs may be referred to as “Advanced-UE's”.
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink. A “symmetric configuration” refers to the case where the number of CCs in downlink and uplink is the same, whereas an “asymmetric configuration” refers to the case that the number of CCs is different in uplink and downlink. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a terminal (or UE): A terminal may, for example, support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs. The link between DL CCs and UL CCs can be UE-specific.
Scheduling of a CC is done on the PDCCH via downlink assignments. Control information on the PDCCH may be formatted as a Downlink Control Information (DCI) message. In Rel-8, a terminal only operates with one DL and one UL CC. Therefore, the association between DL assignment, UL grants, and the corresponding DL and UL CCs is clear in Rel-8. However, in Rel-10, two modes of CA need to be distinguished: The first mode is very similar to the operation of multiple Rel-8 terminals—i.e., a DL assignment or UL grant contained in a DCI message transmitted on a CC is either valid for the DL CC itself or for associated (either via cell-specific or UE-specific linking) UL CC. A second mode of operation augments a DCI message with the Carrier Indicator Field (CIF). A DCI message containing a DL assignment with CIF is valid for that DL CC indicated with CIF and a DCI containing an UL grant with CIF is valid for the indicated UL CC.
It is observed here that it is desirable to control transmit power for a transmit signal (e.g., a PUCCH signal to be transmitted from a UE to a base station) while exchanging data between a base station (BS) and a UE. In particular, transmit power control of an uplink channel is important in terms of power consumption of the UE and service reliability. In uplink transmission, if a transmit power is too weak, the BS cannot receive a transmit signal of the UE. On the other hand, if the transmit power is too strong, the transmit signal may act as interference to a transmit signal of another UE, and may increase battery consumption of the UE transmitting such a powerful signal.
DCI messages for downlink assignments (of uplink resources) contain, among others, resource block assignment, modulation and coding scheme related parameters, HARQ redundancy version, etc. In addition to those parameters that relate to the actual downlink transmission, most DCI formats for downlink assignments also contain a bit field for Transmit Power Control (TPC) commands. These TPC commands may be used by eNB to control the uplink power of the corresponding PUCCH that is used to transmit the HARQ feedback (in response to the received DCI message via PDCCH). More generally, TPC commands are used to control transmit power of a channel between a base station (BS) and a UE.
Each DL assignment may be scheduled with its own DCI message on the PDCCH. Since Rel-8 DCI formats or formats very similar to Rel-8 are also used for Rel-10, each received DCI message in Rel-10 therefore contains a TPC bit field providing an adjustment value for the transmit power for PUCCH. It is observed here that the operating point for all PUCCH formats is common. That is, Rel-8 PUCCH formats 1/1a/1b/2/2a/2b and additional PUCCH formats in Rel-10—i.e., PUCCH format 3 and channel selection based HARQ feedback schemes—all use the same power control loop, with the exception of the power control parameters h(nCQI,nHARQ) and ΔF—PUCCH(F) (defined below with reference to equation (1)). These parameters at least take into account different performance and payload sizes for the different PUCCH formats. Therefore these parameters are individually determined per PUCCH format.
In Rel-8, the PUCCH power control is defined as follows:PPUCCH(i)=min{PCMAX,P0—PUCCH+PL+h(nCQI,nHARQ)+ΔF—PUCCH(F)+g(i)}  (1)In the above equation (1), “PPUCCH(i)” refers to PUCCH transmit power for subframe “i” (e.g., a 1 ms subframe in a 10 ms radio frame); “PCMAX” refers to configured maximum transmit power (at UE) for PUCCH CC (e.g., a UL PCC (Uplink Primary CC)); “P0—PUCCH” refers to desired PUCCH receive power (at eNB or other similar control node in LTE) signaled by higher layers (in an LTE network); “h(nCQI,nHARQ)” refers to an offset parameter that depends on the number “nCQI” (≧0) of CQI bits or the number “nHARQ” (≧0) of HARQ bits (in the PUCCH signal to be transmitted by the UE), to retain the same energy per information bit; “ΔF—PUCCH” refers to an offset parameter that depends on the PUCCH format (of the PUCCH signal transmitted by the UE), to give sufficient room for different receiver (e.g., eNB or other base station) implementation and radio conditions;
  “            g      ⁡              (        i        )              =                  g        ⁡                  (          i          )                    +                        ∑                      m            =            0                                M            -            1                          ⁢                              δ            PUCCH                    ⁡                      (                          i              -                              k                m                                      )                                ”refers to an accumulated power adjustment value derived from TPC command “δPUCCH (i)”. The values “M” and “km” depend on whether the duplexing mode (e.g., the mode of communication between UE and eNB) is Frequency Division Duplex (FDD) or Time Division Duplex (TDD); and “PL” refers to path loss.
It is known that, in Rel-8, PUCCH supports multiple formats such as format 1, 1a, 1b, 2, 2a, 2b, and a mix of formats 1/1a/1b and 2/2a/2b. These PUCCH formats are used in the following manner: PUCCH format 1 uses a single Scheduling Request Indicator (SRI) bit, PUCCH format 1a uses a 1-bit ACK/NACK, PUCCH format 1b uses a 2-bit ACK/NACK, PUCCH format 2 uses periodic CQI, PUCCH format 2a uses periodic CQI with 1-bit ACK/NACK, and PUCCH format 2b uses periodic CQI with 2-bit ACK/NACK.
In Rel-8/9, h(nCQI,nHARQ) is defined as follows:
a. For PUCCH formats 1, 1a and 1b, h(nCQI,nHARQ)=0
b. For PUCCH formats 2, 2a, 2b and normal cyclic prefix
      h    ⁡          (                        n          CQI                ,                  n          HARQ                    )        =      {                                        10            ⁢                                                  ⁢                                          log                10                            ⁡                              (                                                      n                    CQI                                    4                                )                                                                                        if              ⁢                                                          ⁢                              n                CQI                                      ≥            4                                                0                          otherwise                    
c. For PUCCH format 2 and extended cyclic prefix
      h    ⁢          (                        n          CQI                ,                  n          HARQ                    )        =      {                                        10            ⁢                                                  ⁢                                          log                10                            ⁡                              (                                                                            n                      CQI                                        +                                          n                      HARQ                                                        4                                )                                                                                                        if                ⁢                                                                  ⁢                                  n                  CQI                                            +                              n                HARQ                                      ≥            4                                                0                          otherwise                    